Pics would probably help most, but if you got your DHG from a big box store, most likely it was grown emersed and now is going through a major meltdown as it transitions to submerged growth.
I've found dosing Excel to be a major help in keeping algae in check while plants go through this process.
Most people switch to canister filters when they start running CO2 on their tanks to avoid outgassing CO2, but HOB filters can work too as long as they don't have biowheels and you're good about keeping the water levels high enough to avoid surface agitation. AquaClears are a popular choice.
That photo of your turtle.. looks like its doing some sorta sexy dance and giving a "heeeey baaaby" kinda look" quite amusing, shame its so anti.. anything lol I have a 11+" leopard sailfin pleco that's about as bad (but doesn't eat fish thankfully... yet) but he won't let any plants live in hsi tank he eats them all.. so solution: make a 'garden' aka aquaponic system above the tank that canister water goes through! "HAHA CANT REACH THOSE PLANTS!" *ahem*
Im building him a pond as he has out grown this 55 rather quickly. He ate a 7" pleco after 2 years of companionship. It just vanished.... However, when I let him mingle with some younger slider turtles he freaks and hides in the sand only popping his head up to see if they're gone. He is a very wierd creature indeed.